Some recipes e.g. expant generate ninja files and no Makefile may exist,
therefore grep -q would fail on a non-existent file, we just need the
return code to decide if intall-ptest target should be run or not.
Fixes errors like
| grep: Makefile: No such file or directory
(From OE-Core rev: 01f3966d6bf2442a28b460b67bfc02241cd9ddc2)
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>